The invention relates to an expansion body with a spring which is under prestress, and to an SCR system with such an expansion body. Furthermore, the invention relates to a method for monitoring a pressure sensor in such an SCR system. In addition, the invention relates to a computer program which executes each step of the method when it is executed on a computing device, and to a machine-readable storage medium which stores the compute program. Finally, the invention relates to an electronic controller which is configured to execute the method according to the invention.
Nowadays, the SCR (Selective Catalytic Reduction) method is used during the post-treatment of exhaust gases, in order to reduce nitrogen oxides (NOx) in the exhaust gas. The SCR method is primarily used in motor vehicles. A urea-water solution (HWL), also known commercially as AdBlue®, is introduced as a reducing agent solution into the oxygen-rich exhaust gas. For this purpose, a metering valve is used which comprises a nozzle for spraying the urea-water solution into the exhaust gas stream. Downstream of an SCR catalytic converter, the urea-water solution reacts to form ammonia, which subsequently binds with the nitrogen oxides at the SCR catalytic converter, producing water and nitrogen. The metering valve is connected to a feed module via a pressure line. A feed pump of the feed module pumps the urea-water solution from a reducing agent tank to the metering module. In addition, a return line is connected to the reducing agent tank, via which excess urea-water solution can be fed back. An orifice or throttle in the return line controls the return flow.
In order to determine and monitor the pressure in the SCR system, a pressure sensor is provided in the pressure line. In order to ensure the functional capability of the pressure sensor a pitch error is to be detected in addition to an offset error. By means of this detection, the legal requirements relating to the monitoring of what are referred to as “comprehensive components” for on-board diagnostics (OBD) are to be satisfied.
DE 10 2004 022 115 A1 describes a method for performing diagnostics of the pressure sensor. Checking as to whether the pressure sensor signal corresponds at least approximately to the steady-state pressure is provided in at least a first time interval in which a steady-state pressure state is expected. Furthermore, in at least a second time interval, in which a change in pressure occurs, it is checked whether the pressure sensor signal corresponds at least approximately to a predefined test pressure and/or has approximately a predefined change with respect to the steady-state pressure. An error signal is made available if at least one of the conditions is not satisfied. An alternative provides that in the second time interval the difference quotient and/or the differential quotient of the time profile of the pressure signal is determined and evaluated.
In SCR systems, expansion bodies are used, predominantly in order to compensate an increasing volume, and therefore, associated therewith, increasing pressure, if the urea-water solution which is located in the pressure line connected to the expansion body freezes. The expansion body has an equalization chamber which is separated by means of a diaphragm from the pressure line and the urea-water solution which is located therein. If the pressure in the pressure line increases, the diaphragm is pressed in the direction of the equalization chamber, with the result that its volume decreases and a gas enclosed therein is compressed. When the pressure is reduced, the diaphragm returns to its initial position. It is also known to connect a mechanical spring to the diaphragm, with the result that a spring force is applied to the latter. The spring is preferably arranged inside the equalization chamber.